Consideration shall be given to the relationship of the subject
property to the drainage pattern of the watershed. A concentrated
discharge of stormwater to an adjacent property shall be within an
existing watercourse or confined in an easement or returned to a predevelopment-flow-type
condition.

All existing and natural watercourses, channels, drainage systems
and areas of surface water concentration shall be maintained in their
existing condition unless an alteration is approved by the appropriate
regulatory agency.

The invert of all stormwater management facilities and underground
infiltration/storage facilities shall be located a minimum of two
feet above the seasonal high groundwater table or other soil-limiting
zone. The invert of stormwater facilities may be lowered if adequate
subsurface drainage, which does not alter the existing water table
level, is provided.

Any stormwater management facility may be required to be fenced
with a minimum four-foot-high fence of material acceptable to the
City of Harrisburg. Gates with a minimum opening of 10 feet shall
be provided for access.

Storm water management facilities excavated to carbonate rock
must either be fitted with an impervious clay liner, or over-excavated
four feet and refilled with a suitable material mix. Suitable backfill
material is subject to the approval of the Municipal Engineer.

Any stormwater management facility designed to store runoff
and requiring a berm or earthen embankment, shall be designed to provide
an emergency spillway to handle peak rate of stormwater runoff up
to and including the one-hundred-year post-development flow, with
a blocked primary outlet structure. The height of embankment must
be set so as to provide a minimum one foot of freeboard through the
spillway, above the maximum water surface elevation, computed when
the spillway functions for the one-hundred-year post-development inflow,
with a blocked outlet structure. The primary outflow structure must
be designed to pass all design storms (up to and including the one-hundred-year
event) without discharging through the emergency spillway. The maximum
water depth within any stormwater management facility shall be no
greater than eight feet when functioning through the primary outlet
structure.

Emergency spillways shall be armored to prevent erosion during
the one-hundred-year post-development flow, with blocked primary outlet
structure. Synthetic liners or rip-rap may be used, and calculations
sufficient to support proposed armor must be provided. An earthen
plug must be used to accurately control the spillway invert if rip-rap
is the proposed armoring material. Emergency spillway armor must extend
up the sides of the spillway and continue at full width to a minimum
of 10 feet past the toe of slope.

Stormwater management facility berm cross-sections must be at
least five feet wide at the top and eight feet wide through the emergency
spillway. For fill embankments, the side slopes shall be no steeper
than 3:1 on the inside of the facility and 2:1 on the outside of the
facility. For cut slopes, the side slopes shall be no steeper than
2:1.

Anti-seep collars must be provided on all outflow culverts in
accordance with the methodology contained in the latest edition of
the E&S Manual. An increase in seepage length of 15% must be used
in accordance with the requirements for permanent anti-seep collars.

Conventional, non-BMP stormwater management facilities (i.e.,
dry detention basins) must empty over a period of time not less than
24 hours and not more than 72 hours from the end of the facility's
inflow hydrograph. Infiltration tests performed at the facility locations
and proposed basin bottom depths, in accordance with the BMP Manual,
must support time-to-empty calculations if infiltration is a factor
in the sizing of the stormwater management facility.

Impervious low-flow channels are not permitted within stormwater
management facilities to promote water quality and groundwater recharge
for frequent storm events. Facilities designed as water quality/infiltration
BMPs may have a bottom slope of zero. Minimal maintenance, saturation-tolerant
vegetation must be provided in basins designed as water quality/infiltration
BMPs. Conventional, non-BMP stormwater management facilities must
have a minimum slope of 1% extending radially out from the primary
outlet structure. Water storage below the lowest outlet structure
stage (i.e., dead storage) is permitted in stormwater management facilities
designed as water quality/infiltration BMPs.

Stormwater management facility bottom elevations must be greater
than adjacent floodplain elevations (FEMA or HEC-RAS analysis). If
no floodplain is defined, bottom elevations must be higher than existing
ground elevations 50 feet from top of stream bank in the facilities
vicinity.

Basin outflow culverts discharging into floodplains must account
for tailwater. Tailwater corresponding to the one-hundred-year floodplain
elevation may be used for all design storms, or the applicant may
elect to determine flood elevations of the adjacent watercourse for
each design storm. The floodplain is assumed to be 50 feet from top
of stream bank in areas where a floodplain is not designated, or where
no other evidence is provided.

Exceptions to these requirements may be made at the discretion
of the City of Harrisburg for BMPs that retain or detain water but
are of a much smaller scale than traditional stormwater management
facilities.

Storm sewers must be able to convey post-development runoff
from a ten-year design storm without surcharging inlets where appropriate.
When connecting to an existing storm sewer system, the applicant must
demonstrate that the proposed system will not exacerbate any existing
stormwater problems and that adequate downstream capacity exists.

A minimum pipe size of 15 inches in diameter shall be used in
all roadway systems (public or private) proposed for construction.
Pipes shall be designed to provide a minimum velocity of 2 1/2
feet per second when flowing full, but in all cases, the slope shall
be no less than 0.5%. Arch pipe of equivalent cross-sectional area
may be substituted in lieu of circular pipe where cover or utility
conflict conditions exist.

In proposed curbed roadway sections, the maximum encroachment
of water on the roadway pavement shall not exceed half of a through
travel lane or one inch less than the depth of curb during the ten-year
design storm of five-minute duration. Gutter depth shall be verified
by inlet capture/capacity calculations that account for road slope
and opening area. The maximum distance between inlets in curbed roadway
sections shall be no more than 600 feet; however, access to underground
pipes shall be provided every 300 feet.

Standard Type C inlets with eight-inch hoods shall be used along
vertical concrete curb roadway networks. Type C inlets with ten-inch
hoods that provide a two-inch sump condition may be used with approval
of the Municipal Engineer when roadway longitudinal slopes are 1.0%
or less.

On curbed sections, a double inlet shall be placed at the low
point of sag vertical curves, or an inlet shall be placed at the low
point and on each side of the low point at a distance not to exceed
100 feet, or at an elevation not to exceed 0.2 foot above the low
point.

At all roadway low points, swales and easements shall be provided
behind the curb or swale and through adjacent properties to channelize
and direct any overflow of stormwater runoff away from dwellings and
structures.

Inlets must be sized to accept the specified pipe sizes without
knocking out any of the inlet comers. All pipes entering or exiting
inlets shall be cut flush with the inlet wall. A note to this effect
shall be added to the SWM site plan or inlet details therein.

Inlets, manholes, pipes, and culverts shall be constructed in
accordance with the specifications set forth in PennDOT's Publication
408, latest edition, and as detailed in the PennDOT's Publication
72M, Standards for Roadway Construction (RC), latest edition, or as
approved by the Municipal Engineer. All material and construction
details (inlets, manholes, pipe trenches, etc.), must be shown on
the SWM site plan; and a note shall be added that all construction
must be in accordance with PennDOT's Publication 408 and PennDOT's
Publication 72M, latest edition. A note shall be added to the plan
stating that all frames, concrete top units, and grade adjustment
rings shall be set in a bed of full mortar according to Publication
408.

Accessible drainage structures shall be located on continuous
storm sewer system at all vertical dislocations, at all locations
where a transition in storm sewer pipe sizing is required, at all
vertical and horizontal angle points exceeding 5°, and at all
points of convergence of two or more storm sewer pipes.

All storm drainage piping (equal to or greater than 12 inches)
discharging to the ground surface shall be provided with either reinforced
concrete headwalls and end sections or plastic and metal pipe end
sections compatible with the pipe size involved in accordance with
PennDOT Publication 408 and Publication 72M.

Pavement base drain shall be provided at all low point in cut
areas, toe of slope areas, and other areas as dictated by proven engineering
principles and design judgment. All base drain shall be in accordance
with PennDOT Publication 408.

Swales shall be designed for stability using velocity or shear
criteria. Velocity criteria may be used for channels with less than
10% slope. Shear criteria may be used for all swales. Documentation
must be provided to support velocity and/or shear limitations used
in calculations.

Stormwater runoff from all development sites shall be calculated using either the Rational Method or a soil cover complex methodology. Methods selected by the qualified professional shall be approved by the approving agency at any point in the review, based on the individual limitations and suitability of each method for a particular site as stipulated in § 9-907.2D.

Rational Method: The Pennsylvania Department of Transportation
Drainage Manual, Intensity-Duration-Frequency Curves, Publication
584, Chapter 7A, latest edition, shall be used in conjunction with
the appropriate time of concentration and return period.

Soil Cover Complex Method: The Soil Conservation Service Type
II, twenty-four-hour rainfall distribution shall be used in conjunction
with rainfall depths from NOAA Atlas 14 or consistent with the following
table.

Rational Method: may be used for drainage areas up to five acres.
Extreme caution should be used by the qualified professional if the
watershed has more than one main drainage channel, if the watershed
is divided so that hydrologic properties are significantly different
in one versus the other or if the time of concentration exceeds 30
minutes. The combination of Rational Method hydrographs shall be prohibited.

Soil Cover Complex Method: may be used for drainage areas greater
than 20 acres. This method is recommended for design of stormwater
management facilities and where stormwater runoff volume must be taken
into consideration.

For the purposes of predevelopment peak flow rate and volume
determination, 20% of existing impervious area, when present on the
project site and contained within the new proposed limit of disturbance,
shall be considered meadow (good condition) for predevelopment hydrologic
calculations for redevelopment.

All stormwater management facilities shall be verified by routing
the proposed one-year, two-year, ten-year, twenty-five-year, fifty-year,
and one-hundred-year hydrographs through the facility using the storage
indication (Modified Puls) method. The design storm hydrograph shall
be computed using a calculation method that produces a full hydrograph.

The stormwater management and drainage system shall be designed
to safely convey the post-development one-hundred-year storm event
to stormwater detention facilities, for the purpose of meeting peak
rate control.

All structures (culvert or bridges) proposed to convey runoff
under a municipal road shall be designed to pass the fifty-year design
storm with a minimum one foot of freeboard measured below the lowest
point along the top of the roadway.

All design within state or federal rights-of-way or that falls
under the design criteria of any higher authority must meet the requirements
of that agency in addition to meeting the minimum requirements of
this Part 9.

Time of concentration shall be computed using the NRCS Segmental
Method as described in TR-55 (SCS 1986 or most current update). The
length of sheet flow shall be limited to 100 feet. The Manning's
"n" roughness coefficient for TR-55 sheet flow can be found in Table
C-4 (Appendix C).[3] Time of concentration for channel and pipe flow shall
be computed using Manning's equation.

Under extremely unusual circumstances, for sites with insignificant
channelized flow and less than 20% imperviousness coverage, the time
of concentration may be computed using the NRCS equation for lag time:

Undetained fringe areas (areas that are not tributary to a stormwater
facility but where a reasonable effort has been made to convey runoff
from all new impervious coverage to best management practices) may
be assumed to represent the predevelopment conditions for purpose
of time of concentration calculations.

Where uniform flow is anticipated, the Manning's equation shall
be used for hydraulic computations and to determine the capacity of
open channels, pipes, and storm sewers. The Manning's equation
should not be used for analysis of pipes under pressure flow or for
analysis of culverts. Manning's "n" values shall be obtained
from Table C-3 (Appendix C).[4] Inlet control shall be checked at all inlet boxes to ensure
that the headwater depth during the ten-year design event is contained
below the top of grate for each inlet box.

The City of Harrisburg may approve the use of any generally accepted
full hydrograph approximation technique that shall use a total runoff
volume that is consistent with the volume from a method that produces
a full hydrograph.

The City of Harrisburg has the authority to require that computed
existing runoff rates be reconciled with field observations, conditions
and site history. If the designer can substantiate, through actual
physical calibration, that more appropriate runoff and time of concentration
values should be utilized at a particular site, then appropriate variations
may be made upon review and approval of the City of Harrisburg.